Data in optical communication is often encoded by modulation of the intensity of a beam of light. Such amplitude modulation can be achieved by combining a selectively delayed copy of a light beam with itself. When the copy of the light beam is subjected to a phase delay of π radians and combined with the original light beam, destructive interference occurs, yielding minimum output intensity. When the copy of the light beam is subjected to no phase delay, constructive interference occurs, yielding maximum output intensity. An optical phase-shifter device under the control of electrical signals can be used to selectively delay the light beam in accordance with a stream of data, thereby amplitude modulating the output light in accordance with the data.
Various devices have been developed to modulate the intensity of a light beam. Lithium niobate (LiNbO3) modulators can be fast and have reasonable voltage requirements, however, they are not polarization independent and do not lend themselves to integration of drive electronics and optical components. Integrated doped silica waveguides, also known as silicon optical bench components, offer polarization independence and a high degree of integration, however, their highest switching speeds are only in the 1 MHz range. Semiconductor modulators (InP or GaAs) can have a 40 GHz bandwidth; however, polarization independence and extensive integration of multiple channels and other components are not easily achieved with this technology. Silicon modulators consisting of silicon waveguides embedded in silica allow extensive integration; however, designs to date have had a rather low phase change per unit voltage and length, requiring either a high operating voltage or a large device. Many existing designs also dissipate a high degree of static power, for instance, P-I-N devices which have current flowing continuously through the device in order to maintain a steady concentration of carriers.